In PR techniques a pump beam and a probe beam are simultaneously directed at a small spot on a semiconductor wafer. The pump beam, which is absorbed in the semiconductor and is chopped at a relatively slow frequency, generates electron-hole pairs which modulate the built-in electric field of the semiconductor material. The probe beam is reflected from the wafer surface, and the reflected signal is detected to provide information about the material's characteristics. According to F. H. Pollack et al., PR has been known for more than 20 years, but interest in it has been diminished by experimental difficulties including scattered light from the pump beam and photoluminescence from the wafer. Pollack et al. describe computerized procedures to improve the signal-to-noise ratio and gain additional information on the materials examined. In this regard, see the family of F. H. Pollack et al. U.S. Pat. Nos. including 5,260,772 (issued on Nov. 9, 1993), 5,270,797 (issued on Dec. 14, 1993), 5,255,070 (issued on Oct. 19, 1993), 5,255,071 (issued on Oct. 19, 1997) and 5,287,169 (issued on Feb. 15, 1994), all of which are incorporated herein by reference.
The prior art procedures implementing such PR techniques have been slow and cumbersome, entailing measurements made at only one small spot at a time on a wafer. Each such measurement typically consumes 15-30 minutes. Thus, it could take 50-100 hours to make several hundred PR measurements on even a relatively small 2-inch diameter wafer (as would done to map the entire wafer). Of course, mapping of even larger wafers would take commensurately longer.
Thus, a need remains in the art for a PR technique which improves throughput by enabling multiple measurements, and hence wafer mapping, to be performed more rapidly.